Conventionally, a magnetic core having a gap used for the transformer and the choke coil, has been utilized as follows: an opposed pair of E-shaped cores made from magnetic materials such as ferrite, which is disclosed in Japanese Patent Publication No. 50372/1980, wherein gaps are provided in the end of a center leg so as to prevent magnetic saturation, or a combination of E-shaped and I-shaped core which is disclosed in Japanese Patent Publication No. 24363/1981, wherein the similar gaps as described above are provided in the end of the center leg of the E-shaped core.
However, such conventional magnetic cores having gaps, used in a choke coil or a transformer having a wire wound on the center leg often cause errors in inductance, which is most often derived from errors in the dimension of the magnetic core, errors during the production of the gaps and errors in magnetic permeability of the core. For example, when an effective permeability is 100, the variation of the inductance of the choke coil is .+-.21% in E--E type (disclosed in Japanese Patent Publication 50372/1980), and is .+-.16% in E-I type (disclosed in Japanese Patent Publication 24363/1981).